A `;`-separated CSV (the usual CSV where `,` is the decimal separator) was
rewritten with `,` on save: read_csv sniffed the delimiter but the result was
dropped, and save_bytes passed a hard-coded b','. file.save, Save As .csv,
file.exportCsv, file.saveBytes and gridcraft-cli convert all went through it.
The workbook now remembers the separator it was read with
(Workbook::csv_delimiter, set by read_csv) and a CSV save writes it back.
Workbooks not read from CSV, and .tsv/.txt sources saved as .csv, keep `,`.
Regression tests: `;`, `,` and tab .csv round trips byte for byte, new and
.xlsx-origin workbooks save with `,`, and open-by-path + edit + file.save on a
`;` file keeps `;` (also through exportCsv and saveBytes).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
b13ea3e2f3